The present invention relates to a method for treating a human with human immunodeficiency virus infection. The method comprises administering to the human a therapeutically effective amount of a thymidine analog, which analog acts as an inhibitor of viral reverse transcriptase necessary for viral replication of human immunodeficiency virus, and a thymidylate synthase inhibitor. In other embodiments, the method further comprises administering to the human a therapeutically effective amount of a folate antagonist or hydroxyurea, or both.
The disclosures referred to herein to illustrate the background of the invention and to provide additional detail with respect to its practice are incorporated herein by reference. For convenience, the disclosures are referenced in the following text and respectively grouped in the appended bibliography.
Acquired immunodeficiency syndrome (AIDS) is believed to be caused by the human immunodeficiency virus (HIV). Human immunodeficiency virus is a retrovirus which replicates in a human host cell. The human immunodeficiency virus appears to preferentially attack helper T-cells (T-lymphocytes or OKT4-bearing T-cells). When the helper T-cells are invaded by the virus, the T-cells become a human immunodeficiency virus producer. The helper T-cells are quickly destroyed causing the B-cells and other T-cells, normally stimulated by helper T-cells, to no longer function normally or produce sufficient lymphokines and antibodies to destroy the invading virus or other invading microbes.
Although the human immunodeficiency virus does not necessarily cause death, the virus generally causes the immune system to be so depressed that the human develops secondary infections such as herpes, cytomegalovirus, pneumocystis carinni, toxoplasmosis, tuberculosis, other mycobacteria, and other opportunistic infections. Kaposi""s sarcoma, lymphomas, and cervical cancer may also occur. Some humans infected with the human immunodeficiency virus appear to live with little or no symptoms, but appear to have persistent infections, while others suffer mild immune system depression with symptoms such as weight loss, malaise, fever, and swollen lymph nodes. These syndromes have been called persistent generalized lymphadenopathy syndrome (PGL) and AIDS related complex (ARC) and generally develop into AIDS. Humans infected with the AIDS virus are believed to be persistently infective to others.
Human immunodeficiency virus is an extremely heterogeneous virus. The clinical significance of this heterogeneity is evidenced by the ability of the virus to evade immunological pressure, survive drug selective pressure, and adapt to a variety of cell types and growth conditions. A comparison of isolates among infected patients has revealed significant diversity, and within a given patient, changes in the predominant isolate over time have been noted and characterized. In fact, each patient infected with human immunodeficiency virus harbors a xe2x80x9cquasispeciesxe2x80x9d of virus with a multitude of undetected viral variants present and capable of responding to a broad range of selective pressures, such as those imposed by the immune system or antiviral drug therapy. Therefore, diversity is a major obstacle to pharmacologic or immunologic control of human immunodeficiency virus infection. Human immunodeficiency virus infection has multiple mechanisms to maximize its potential for genetic heterogeneity. These mechanisms result in an extremely diverse population of virus capable of responding to a broad range of selective pressures, including the immune system and antiretroviral therapy, with the outgrowth of genetically altered virus.
When a patient with human immunodeficiency virus infection is initiated on antiretroviral therapy, there is generally a virologic response characterized by declining viremia and antigenemia (5,7,19,20,25). Unfortunately, the currently available antiretroviral agents which have undergone clinical evaluation have only limited benefit because most patients will ultimately have evidence of worsening disease and increasing viral burden. This progression often occurs in association with the emergence of drug-resistant human immunodeficiency virus. For example, most patients who are treated with 3xe2x80x2-azido-3xe2x80x2-deoxythymidine (AZT) will have initial evidence, of improvement of clinical and laboratory parameters of human immunodeficiency virus infection (7,20). The duration of this benefit varies from patient to patient and is likely to be disease stage related (21). Ultimately, however, most patients will have progressive disease and genotypic or phenotypic evidence of the appearance of AZT-resistant human immunodeficiency virus (9,12). Since clinical failure and the appearance of virus with high level resistance to AZT both occur with evidence of increasing levels of viremia and changes in viral tropism, it has been difficult to ascribe the clinical failure solely to the development of AZT resistance (2,11). Nevertheless, it seems likely that AZT resistance ultimately contributes to the clinical failure seen in most patients receiving prolonged AZT therapy.
While the development of viral-encoded drug resistance may contribute to the limited efficacy of currently used antiretroviral agents, it cannot explain all of the in vitro and in vivo phenomena associated with viral replication in the presence of an antiretroviral agent. For example, many patients will have continued evidence of viral replication after initiation of AZT therapy, but the isolated virus will remain sensitive to AZT when analyzed in tissue culture (7,20). In contrast, high level human immunodeficiency virus resistance to many of the non-nucleoside reverse transcriptase inhibitors develops very rapidly in culture and in patients (13,16,22,23). Some of these differences may relate to the complexity and prevalence of viral variants harboring pre-existing drug resistance mutations prior to the application of the selective pressure. However, some of the differences may be due to cellular heterogeneity in the uptake or metabolism of the antiretroviral agents, that is, each cell population may have some cells that are refractory to the antiviral effects of the drug. This would allow a subset of the cellular population to be successfully infected by genetically drug-sensitive human immunodeficiency virus in the presence of the antiviral drug. Depending upon the prevalence of drug-resistant human immunodeficiency virus in the initial population, the relative rates of replication of drug-resistant and drug-sensitive virus, and the percentage of cells refractory to the antiviral effects of the drug, different patterns of viral breakthrough would emerge. Notably, the non-nucleoside reverse transcriptase inhibitors do not undergo cellular metabolism and cellular effects of uptake or metabolism may be less likely in this setting. This is consistent with the observation that viral-encoded drug resistance to the non-nucleoside reverse transcriptase inhibitors develops very rapidly under selection in tissue culture and in patients. In fact, the rapid, development of resistance in patients suggests that the blood and plasma compartment of virus is subjected to drug selective pressure. The presence of human immunodeficiency virus, but lack of AZT-resistant human immunodeficiency virus, early after the initiation of AZT suggests that a component of this viral pool may be capable of averting selective drug pressure in vivo. Continued viral replication in cells in which, AZT is an ineffective antiretroviral agent could conceivably result in the continued growth of virus that is sensitive to AZT. An increase in the number of these cells over time could also alter viral growth kinetics in the presence of AZT without the emergence of virus with high level AZT resistance. Therefore, many mechanisms may contribute to the inability of an antiviral agent to completely suppress human immunodeficiency virus infection. Viral growth in the presence of the non-nucleoside reverse transcriptase inhibitors appears due to the rapid selection of genetically resistant virus. In contrast, genetic viral drug resistance does not appear to be the major mechanism contributing to early viral growth in the presence of AZT.
The use of recombinant human immunodeficiency virus encoding reporter genes has been reported to analyze viral breakthrough infection in the presence of antiretroviral agents (26). In that study, to determine the prevalence of viral variants spontaneously resistant to the non-nucleoside reverse transcriptase inhibitor TIBO R82150, HeLa-T4 cells were infected in the presence of drug with replication defective HIV-gpt (18,26) or HIV-LacZ (26). The recombinant virus used for these infections was produced by infection of cell lines containing an integrated copy of the defective recombinant virus with replication-competent human immunodeficiency virus. The replication-competent human immunodeficiency virus provided the necessary gene products to rescue the defective virus. The prevalence of viral variants containing mutations encoding resistance to TIBO R82150 was reflected by the prevalence of recombinant viruses capable of infecting HeLa-T4 cells in the presence of TIBO R82150. The presence of reporter genes in the recombinant viruses allowed for a quantitative analysis of a single cycle of infection on a single cell basis.
U.S. Pat. No. 4,724,232 (Rideout et al.) discloses a method for treating a human having acquired immunodeficiency syndrome which comprises administering to the human 3xe2x80x2-azido-3xe2x80x2-deoxythymidine.
Cancer, Dec. 15, 1992, vol. 70, No. 12, pp. 2929-2934 (Posner et al.) discloses the use of 3xe2x80x2-azido-3xe2x80x2-deoxythymidine and 5-fluorouracil in the treatment of cancer.
The measurement of plasma HIV RNA copy number after the initiation of antiviral therapy has provided several insights into the kinetics and dynamics of HIV infection. Initial studies quantitating HIV RNA after the initiation of a non-nucleoside reverse transciptase inhibitor (NNRTI), nevirapine, indicated a very rapid turnover of plasma HIV (28). In those studies there was an initial decline in HIV RNA followed by a rapid increase in plasma viral RNA. The studies with nevirapine demonstrated that the rapid rebound in HIV RNA levels was a consequence of the outgrowth of HIV with, phenotypic and genotypic resistance to nevirapine (28). An in vitro model of HIV infection after the initiation of a different NNRTI (TIBO) has also indicated a similarly high prevalence of variants capable of infection in the presence of the drug (26).
Similar clinical and laboratory studies analyzing early HIV infection in the presence of AZT have also been undertaken (29). In contrast to the clinical studies with nevirapine, early HIV infection in the presence of AZT does not appear to be predominated by the early outgrowth of drug-resistant HIV. While the amount of virus circulating in plasma shortly after the initiation of AZT rapidly declines, the remaining circulating virus after this decline does not contain mutations known to encode resistance to AZT (29). Laboratory infections using an in vitro model of infection in the presence of AZT have demonstrated a similar pattern: early breakthrough infection independent of the presence of genetic resistance (31). These more complex dynamics may be a consequence of a variety of pharmacologic, cellular and viral features. The mutations associated with AZT-resistance may be present in the initial (unselected) viral population but mutant HIV with high level AZT-resistance generally contain multiple mutations associated with AZT-resistance and the emergence of these variants often occurs over months-years. While the slow emergence of these high level resistant variants can be explained by a low prevalence of AZT-resistant variants, the need for superimposed mutations, or selection against the emergence of these variants, the early outgrowth of AZT-sensitive virus in the presence of AZT must be explained by virologic, cellular or pharmacologic features that result in the ability of HIV-1 that is genotypically and phenotypically sensitive to AZT to replicate in the presence of AZT.
A quantitative in vitro model of HIV infection which utilizes recombinant HIV has been used to characterize some of the mechanisms responsible for HIV kinetics after the initiation of antiviral drugs (26,31). In that model a replication-defective HIV encoding a selectable marker is used to assess a single cycle of infection in the absence of either repeated cycles of infection or selection of virus in the presence of antiviral drugs. The use of a replication-defective virus allows an assessment of mechanisms of early HIV breakthrough infection in the presence of antiviral drugs and has been used to quantitate HIV breakthrough infection. Similarly, this system has been used to determine that such infection in the presence of a NNRTI is likely due to infection by genetically resistant virus while early infection in the presence of AZT is due to the infection by virus without genetic drug resistance (26,31). These in vitro results mimic those described in clinical studies of HIV dynamics after the initiation of a NNRTI (28) or AZT (29).
Another feature of the replication-defective recombinant HIV system is that cells infected with HIV in the presence of the antiviral drug can be readily isolated and characterized. Using this approach it has been possible to determine that some of the cells infected in the presence of AZT had metabolic features that rendered AZT an ineffective antiviral drug. Attempts to reverse these metabolic features has resulted in the development of new drug combinations designed to modulate the antiviral efficacy of AZT. One such combination has improved antiviral efficacy in both cells demonstrated to be refractory to the antiviral effects of AZT and primary blood mononuclear cells (35).